The invention is supported by the provisional application U.S. 60/577,094 filed on Jun. 4, 2004.
The invention relates to an image sensor, and in particular, to a CMOS image sensor.
Image sensors convert light to digital signals. A typical image sensor comprises a pixel sensor array. An image is brought into focus on the image sensor via lenses, and the pixel sensors convert the image into digital signals for data processing.
CMOS image sensors, in comparison to conventional image sensors, are cost effective and compact. Specifically, CMOS image sensors comprise several advantages including low operating voltage, low power consumption, random accessibility of image data, CMOS logic design compatibility, and simplified camera design with a single chip. The pixel sensors are referred to as active pixel sensor (APS). An APS includes a plurality of transistors, a photodiode and a current generator. For example, a typical three-transistor APS comprises a reset transistor, a source follower transistor, a row selection transistor, a photodiode and a current generator.
The ratio of maximum and minimum intensity detectable by the image sensor is referred to as a dynamic range (measured in DB). A higher dynamic range may provide better illumination resolution. The human eye is capable of sensing a wide range of illumination, and when intensity is high, the eye automatically decreases it sensitivity. Conventional image sensors, however, may be saturated when the intensity is high, causing image distortion. This phenomenon is referred to as machine mode. The saturation limits the dynamic range of the image sensor, making it difficult to simulate human eye sensibility.